A Total Hip Replacement (THR) involves replacement of both the acetabulum and the femoral head. Prosthesis for a total hip replacement thus consists of a femoral component and an acetabular cup. The femoral component comprises a femoral head (ball) which is attached to a femoral stem that is located in a patient's femur. The femoral component may be modular wherein separate femoral head and stem components are joined together. The stem commonly includes a tapered element, extending from a neck of the stem, for engagement in a complementary recess within the femoral head. This construction is desirable since it allows the head and stem portions to be made from different materials which are optimized for their particular function. Femoral stems are typically made of metals such as cobalt chromium, stainless steel and titanium alloy, which are cheap and resistant to scratching and fracture. Femoral heads may also be made from these materials, or they may be ceramic. For example, it is common to put a cobalt chromium head on a titanium alloy stem using most commonly a 12/14 mm neck taper.
Small femoral heads (typically having an outer diameter of 22 mm) were used in the first successful THR procedures pioneered by Sir John Charnley in the 1960's. At this time, femoral heads were generally metallic and the bearing surface of the acetabular cup was formed from conventional Ultra High Molecular Weight Polyethylene (UHMWP). Although small heads were found to have low wear against such conventional polyethylene cups, they had a high dislocation rate, which was reduced by the use of larger femoral heads (i.e. of 26 mm or more outer diameter). However, it was subsequently discovered that larger heads produced larger volumes of polyethylene debris, when articulating against conventional polyethylene, due to wear of the cup surface and such debris was thought to lead to observed tissue reactions and acetabular and femoral bone loss (known as osteolysis).
In the past 10 years, conventional polyethylene acetabular bearing surfaces have been abandoned in favour of cross-linked polyethylene to reduce cup wear in THRs.
Having largely conquered cup wear by a switch to cross-linked polyethylene, in the past 5 years surgeons have started using bigger and bigger femoral heads against cross-linked polyethylene to reduce the risk of dislocation discussed above in relation to small heads.
Over the past 2 years a new problem of accelerated severe taper corrosion and wear at the head/neck femoral junction has emerged leading to soft tissue reactions (known as pseudotumours).
It is therefore an object of the present invention to provide a femoral prosthesis with optimum characteristics to minimise the risk of pseudotumours and dislocation.